This invention generally relates to machining apparatus and more particularly relates to an adjustable machining apparatus for machining a cylindrical workpiece, such as an electrical generator rotor or stator.
Before addressing the current state of the art, it is instructive first to briefly discuss the structure and operation of a typical electrical generator. In this regard, an electrical generator is part of a coupled turbine-generator set that converts the mechanical power of the turbine into electricity. More specifically, steam is piped to the turbine where it impinges a plurality of turbine blades surrounding and attached to a rotatable turbine shaft. The force of the steam impinging the turbine blades produces a reactive force that is transferred from the turbine blades to the turbine shaft for rotating the shaft. In this manner, the steam imparts mechanical power to the turbine shaft. As the turbine shaft rotates, the mechanical power imparted to the shaft is transferred to the generator because the turbine shaft is coupled to the generator. As discussed more fully hereinbelow, the generator converts this mechanical power into electricity.
The generator includes a rotatable cylindrical rotor shaft forging having a plurality of rotor teeth integrally formed therewith around the circumference of the rotor shaft. Coils of wire are interposed between the rotor teeth for conducting electricity and wedges are press fit between the rotor teeth and placed on top of the wire coils for transmitting the centrifugal load of the coils to the rotor teeth when the rotor shaft is rotating. The rotor shaft is thus constructed such that the wire coils are fixed in position on the rotor shaft by the rotor teeth and wedges.
Another set of wire coils is embedded in the surface of an axial bore that extends through a stationary cylindrical stator. The rotor shaft is freely rotatably disposed in the axial bore of the stator and rotates as the turbine shaft rotates because the rotor shaft is coupled to the turbine shaft. In order to electrically excite the rotor coils, direct current is fed to the rotor coils typically through brushes contacting an end of the rotor shaft. As well understood in the art, a magnetic field is produced around the rotor shaft by the excited rotor coils when the rotor shaft rotates. This magnetic field passes through the wire coils embedded in the stator for inducing an electrical voltage in the stator wire coils. The electrical voltage induced in the stator coils is fed to an external electrical system for supplying electricity to a connected system load.
The rotor teeth themselves are machined into the rotor forging and are designed to have a uniform density and also are intended to be uniformly annealed during the forging process to avoid undesirable internal strains. However, on occasion some of the rotor teeth may contain flaws due, for example, to localized inhomogeneities or non-uniform annealing. Such flaws ultimately could lead to a phenomenon known in the art as stress corrosion cracking during generator operation. As the rotor shaft rotates at high speed, centrifugal force is developed in the rotor teeth that are formed around the circumference of the rotor shaft. This centrifugal force in combination with stress corrosion cracking may result in the flaw or crack propagating through a rotor tooth to cause a portion of the rotor tooth to separate from the rotor shaft and become a high-velocity missile that might impact and damage the generator internals. Of course, such flaws may also develop in the stator teeth. Therefore, a suitable nondestructive examination technique, such as ultrasonic examination, is used to inspect the rotor and stator teeth for flaws and cracks. Of course, once such flaws and cracks are discovered, they are removed, such as by milling.
A prior art machining device has been used to repair cracked rotor teeth. In this regard, the rotor is removed from the generator and disposed adjacent the machining device and rotated to bring any cracked rotor tooth into alignment with a cutting head belonging to the machining device. The machining device is then operated in a manner to mill or remove cracks from the rotor tooth. This process requires the rotor to be repeatedly rotated or adjusted to machine a plurality of cracked rotor teeth because the machining device is designed to be stationary. Also, because the machining device is stationary, the rotor also has to be translated or adjusted along its longitudinal axis so that the machining device can mill cracked rotor teeth located at various longitudinal portions of the rotor.
Although the above recited prior art machining device satisfactorily machines cracked rotor teeth, it requires the rotor to be repeatedly repositioned and adjusted in order to align the machining device's cutting head with a plurality of cracked rotor teeth. Such repositioning and readjustments prolonged the machining operation. Moreover, such a machining device is not suitable for machining cracked stator teeth located in the bore of the stator.
Therefore, what is needed is an adjustable machining apparatus for machining a cylindrical workpiece, which machining apparatus obviates the need to repeatedly adjust or reposition the workpiece in order to machine it.